The interaction of a chemical carcinogen with the genome of a target cell is studied by 1) examining the binding of benzo(a)pyrene to SV40 minichromosomes; 2) examining the binding of benzo(a)pyrene to polytene chromosomes and 3) using antibodies produced by benzo(a)pyrene modified DNA. The results indicate that 1) the packing of the cellular DNA into the nucleosomal conformation does not significantly affect the binding of the carcinogen to DNA. 2) The in vivo incorporation of benzo(a)pyrene adducts into polytene chromosomes do not reveal "hot spots" for carcinogen binding. 3) The binding of BPDE-1 to the genome can be visualized by immunofluorescence and by immunoelectron microscopy.